1. Field of the Invention
The present invention relates to an electrostatic chuck, in which preferably polyimide is used as the dielectric material and in which the resistance to plasma is increased by achieving a very flat surface, as well as an electrostatic chuck which, by virtue of careful pretreatment, production of a diffusion-permeable electrode and modification of the adhesive, can be used at temperatures up to 230.degree. C. Furthermore, by partial modification of the dielectric constant of the material used as the dielectric, it is possible to adjust the force (clamping force) exerted on the substrate (wafer).
2. Description of the Related Art
A great deal of work has already been done on electrostatic chucks in which the electrode(s) (conductor(s) connected to the voltage source) is (are) substantially insulated by an organic polymer.
Although the patent literature makes mention also of other polymers, it is mainly polyimide that is meant, because of the many advantages (plasma resistance, lack of particles, temperature stability, relatively high dielectric constant and high electrical breakdown strength) which it offers. A wide range of systems is proposed, but they all suffer from many disadvantages. In particular these systems have poor temperature stability.
Although individual organic polymers, in particular polyimide, are stable under vacuum and/or protective gas conditions up to temperatures of more than 400.degree. C., the adhesives used to bond the electrodes, insulating layers and pedestal fail at much lower temperatures, mostly in the range of approximately 100 to 150.degree. C.
The state of the art is described in particular in the following patents: U.S. Pat. No. 5,691,876 proposes an adhesiveless, electrostatic chuck for use at high temperatures; however, this chuck is extremely costly to manufacture.
With regard to the concept outlined in the aforementioned patent, it must be stated that, although under favorable conditions this electrostatic chuck can resist temperatures in the intended range from 200.degree. C. to 400.degree. C., given the residual moisture content that is normally present in polyimide foil (up to approx. 4% according to the data sheet provided by the DuPont company) the water vapor generated when the chuck is used at the aforementioned temperatures destroys any bond between a flat metal electrode, acting as a diffusion barrier, and polyimide and/or the pedestal.
U.S. Pat. No. 5,708,557 proposes increasing the puncture resistance of the insulating layer by using a fibre filling.
EP 0 693 771 B1, corresponding to U.S. Pat. No. 5,606,485, describes a step between 1 and 10 .mu.m high provided at the edge of the system in order to improve its resistance to erosion in the plasma.
U.S. Pat. No. 5,634,266 describes an electrostatic chuck in which an erosion-proof smooth surface is achieved by means of a recess provided in the pedestal.
U.S. Pat. No. 5,625,526 describes, among other things, a system in which an insulating polyimide film is produced from the solution.
U.S. Pat. No. 5,560,780 and EP 0 635 869 A1 describe a system in which an electrostatic chuck consisting of polyimide foils is protected by a layer of ceramic material.
EP 0 260 150 A2 describes an electrostatic chuck in which the outer insulating layer of the electrode is made from polyimide.
The following patents also refer to electrostatic chucks consisting at least partially of polyimide, but the chuck system itself, consisting of an organic dielectric and a metallic electrode, has not been significantly improved as regards temperature stability and plasma resistance.
U.S. Pat. No. 5,636,098 and EP 0 742 588A2 describe a system in which the electrostatic chuck is sealed off from the plasma by a barrier.
U.S. Pat. No. 5,486,975 describes a "protective ring" that protects the electrostatic chuck from the plasma.
EP 0 668 608 A1 describes the electrical connection of an electrostatic chuck.
EP 0 668 607 A1 describes a system in which the electrostatic chuck is improved by additionally supplying a protective gas at the edge of the system.
U.S. Pat. No. 5,528,451 describes an improved method of supplying power at the periphery of the system.
EP 0 805 487 A2 describes an electrostatic chuck in which the electrode is highly segmented. When one segment fails, it is disconnected from the power supply and the system continues to be operated with the other segments.
EP 0 734 052 A1 describes an electrostatic chuck that can be operated in both unipolar and bipolar mode.
In addition to the aforementioned system, U.S. Pat. No. 5,671,117 also embodies rear face cooling via a plurality of non-uniformly distributed gas supply holes.
U.S. Pat. No. 5,646,814 describes an electrostatic chuck in which two electrodes are integrated one over the other, one electrode being used to hold the system onto the pedestal, while the second electrode holds the substrate.
U.S. Pat. No. 5,631,803 describes an electrostatic chuck in which on top of the upper foil, which forms the insulating layer, there is mounted a second foil in which grooves are provided to serve as channels for distributing a cooling gas.
U.S. Pat. No. 5,460,684 describes an electrostatic chuck having a defined surface roughness. Polyimide is a possible choice of dielectric material.
U.S. Pat. No. 5,592,358 describes an electrostatic chuck in which a magnet is incorporated into the pedestal in order to achieve a more uniform plasma at the surface of the substrate.
U.S. Pat. No. 5,644,467 describes an electrostatic chuck having a thickened dielectric layer around the cooling gas inlets in order to increase the lifetime of the system. In the case of the spray-applied ceramic layers which are also described, instructions are also given on how this is accomplished, but no such explanation is given for the polyimide, which is also mentioned as a dielectric material.
U.S. Pat. No. 5,328,311 describes an electrostatic chuck in which the pedestal used is cooled by liquid nitrogen and its temperature is controlled by a heating system. The electrostatic chuck consists of polyimide, copper, polyimide.
U.S. Pat. No. 5,275,683 describes an electrostatic chuck in which the edge of the system is curved downwards.
U.S. Pat. No. 5,191,506 describes an electrostatic chuck consisting, among other things, of polyimide, copper, polyimide.
U.S. Pat. No. 5,255,153 describes an electrostatic chuck consisting, among other things, of polyimide, copper, polyimide.
U.S. Pat. No. 4,645,218 describes an electrostatic chuck consisting, among other things, of polyimide, copper, polyimide.
EP 0 467 390 A1 describes an electrostatic chuck in which a liquid-cooled pedestal is used. The electrostatic chuck consists of polyimide, copper, polyimide.
Electrostatic chucks having an organic dielectric (e.g. polyimide) exhibit three fundamental problems that limit the lifetime as well as the potential uses of such chucks. These problems are:
a) In a plasma environment the chucks have a much lower service life than, for example, ceramic systems. This can lead to early failure and thus to equipment downtime. PA0 b) The chucks have a much lower temperature stability than, for example, ceramic systems. PA0 c) In many cases, the temperature distribution of a substrate on the electrostatic chuck is insufficiently homogeneous.